Energy & Fuels 2008, 22, 2789–2795
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Sulfur Abatement in Pyrolysis of Straw Pellets Roger A. Khalil,*,† Morten Seljeskog,‡ and Johan E. Hustad† Norwegian UniVersity of Science and Technology, NO-7491, Trondheim, Norway, and Sintef Energy Research, NO-7465, Trondheim, Norway ReceiVed February 19, 2008. ReVised Manuscript ReceiVed April 10, 2008
A batch-type reactor was used to study the sulfur release from pellets made of Danish wheat straw during inert conditions. In addition, two means of sulfur abatement were investigated, the use of calcium-based additives for sulfur retention and the use of nonthermal plasma (NTP) for the treatment of the devolatilized products. The results were interpreted by quantifying the releases of hydrogen sulfide (H2S) and carbonyl sulfide (COS) to the gas phase for the pyrolysis temperatures of 400-800 °C. In experiments where additives were used, straw was mixed and pelletized with either calcium oxide (CaO) or calcium hydroxide [Ca(OH)2] at different calcium/sulfur ratios. The effect of additives on the sulfur release was difficult to assess by solely investigating the gaseous products. However, a comparison of the sulfur content in the starting material and the char residues after pyrolysis showed evidence of minor sulfur retention. The nonthermal plasma reactor system was set up to process the produced gas either before or after the removal of the liquid fraction from the devolatilized products. Furthermore, carbon dioxide was mixed with the nitrogen carrier gas to achieve a more complex gas composition for the nonthermal plasma. For comparative reasons, some preliminary experiments were performed to study the removal efficiency of H2S in pure nitrogen. The removal efficiency for sulfur under the effect of nonthermal plasma in pure N2 increased with an increasing H2S concentration and plasma power. The highest registered removal efficiency was close to 96%. The removal efficiency of H2S in the pyrolysis experiments was highest at a pyrolysis temperature of 400 °C, with the nonthermal plasma reactor placed downstream of liquid removal. At this configuration, 86% of H2S was removed from the devolatilized products. The best plasma reactor placement was proved to be downstream of liquid removal for both H2S and COS. Increasing the CO2 amount in the carrier gas has improved the removal efficiency of H2S at the cost of increased COS formation.
Introduction Biomass gasification for electricity production is environmentally a better alternative than fossil fuels because biomass is a natural fuel resource that does not contribute to the increase of CO2 emission to the atmosphere. Whether the produced gas is intended for use in combination with a gas turbine or a fuel cell, the sulfur content in such fuels can cause severe problems. It has been reported that H2S, even at a concentration of 10 ppm, can greatly shorten the life span of the catalyst in the solid oxide fuel cell (SOFC).1 For H2S, different sorbents have been successfully used for reducing its concentration to a level that is acceptable for good system operation. However, for sorbents to maintain a high cleaning efficiency, they need to be constantly regenerated, which in return will increase the process complexity along with the costs. The focus of this paper is to study the abatement of sulfur through two different methods, the use of additives for sulfur retention in the ash and processing the devolatilized products with nonthermal plasma (NTP). Sulfur in Straw. Sulfur in the straw is usually assimilated by the roots as inorganic sulfate and transported to the leaves, where a reduction process to sulfide occurs. Sulfide is combined with organic molecules to form cysteine, which is an amino acid found in most proteins. Because this process is continuously occurring in the plant, sulfur can have two forms, organically * To whom correspondence should be addressed. E-mail: roger.a.khalil@ sintef.no. † Norwegian University of Science and Technology. ‡ Sintef Energy Research. (1) Kuramochi, H.; Wu, W.; Kawamoto, K. Fuel 2005, 84, 377–387.
bound and inorganic sulfate.2 It is believed that the organically bound sulfur has a lower stability, which will result in decomposition at low temperature (400 °C) during the devolatilization period. The inorganic sulfates are more stable and will not be released during the devolatilization stage. The ratio of organic to inorganic sulfate as well as the overall sulfur content depend upon the growth conditions and the sulfur supply to the plant during the growth period. Because of the incorporation of the sulfur in the building structure of the plant, removing it in a pretreatment step, such as aqueous leaching, is more difficult compared to other compounds.3 Sulfur Release through Thermal Decomposition. The reaction mechanism of sulfur release during the thermal decomposition of straw depends upon the different fractions of the trace elements that are found in the fuel. Potassium (K) and calcium are the main elements that will influence the release of sulfur to the gas form, while chlorine (Cl) has an indirect effect on the retention of sulfur during thermal decomposition as well. To better understand the behavior of the different elements during thermal degradation, it is important to have an idea on how they are incorporated into the straw. K and Cl remain usually in ionic form K+ and Cl- and are not metabolized by the plant. These compounds will therefore precipitate when the plant is dried and are easier to remove by aqueous leaching. Their main function in the plant is to maintain a neutral charge (2) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385–1399. (3) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860–870.
10.1021/ef8001235 CCC: $40.75 2008 American Chemical Society Published on Web 05/30/2008
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and pH value as well as regulate the osmotic pressure and stimulate enzyme activity. K is an excellent element for sulfur retention under combustion conditions because it is able to form potassium sulfate (K2SO4), which is thermally stable at temperatures above 1000 °C. However, K will be favorably released as potassium chloride (KCl) in cases where biomass contains high Cl concentrations.2 Studies performed on alkali metal emission have shown that Cl starts devolatilization at low temperatures, and as much as 60% will be released between 200 and 400 °C,4 while the rest of the fraction reacts at higher temperatures to form KCl. Silicon (Si) in biomass also has an indirect influence on sulfur retention.5 High concentrations of silicates in the fuel lowers the thermal stability of the formed calcium sulfate (CaSO4) and K2SO4. At temperatures above 700-800 °C and in silicate-rich fuels, the captured sulfur is re-released by following the reaction routes 1 and 2. CaSO4(s) + SiO2(s) T CaO · SiO2(s) + SO3(g)
(1)
K2SO4(s) + SiO2(s) T K2O · SiO2(s) + SO3(g)
(2)
Use of Calcium-Based Additives for Sulfur Retention. For biomass, some investigation has been performed on the retention of sulfur in the ash in biofuel types, such as straw.6–9 It has been reported that the sulfur release during the devolatilization of straw comes from the organically bound sulfur at very low temperatures (200-400 °C) and is difficult to retain.8 Such conclusions were drawn while investigating calcium-based additives on the combustion of straw. Under these experiments, the devolatilization temperature (1200 °C) was well above the pyrolysis temperatures presented in this paper. While retaining the sulfur in the ash under combustion has resulted in many investigations, little has been done on the retention of sulfur in biomass at inert conditions. The retention of sulfur is usually possible in the presence of elements such as calcium that may be able to bind it under proper conditions. The use of calciumbased additives in coal combustion has already shown some promising results in improving their ash-binding ability.10–13 Thus far, the same has not been done for biomass because of the relative lower concentration of sulfur compared to coal. During the pyrolysis, it is expected that sulfur will be released as either H2S or COS, which even at low concentration can cause major problems. These compounds can be expected to bind with the calcium oxide and calcium hydroxide as follows: CaO + H2S T CaS + H2O
(3)
CaO + COS T CaS + CO2
(4)
Ca(OH)2 + H2S T CaS + 2H2O
(5)
Ca(OH)2 + COS T CaS + CO2 + H2O
(6)
(4) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280–1285. (5) Lang, T.; Jensen, P. A.; Knudsen, J. N. Energy Fuels 2006, 20, 796– 806. (6) Wolf, K. J.; Smeda, A.; Muller, M.; Hilpert, K. Energy Fuels 2005, 19, 820–824. (7) Nordin, A. Fuel 1995, 74, 615–622. (8) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Dam-Johansen, K. Energy Fuels 2005, 19, 606–617. (9) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Frandsen, F. J.; DamJohansen, K. Energy Fuels 2004, 18, 810–819. (10) Guan, R.; Li, W.; Li, B. Fuel 2003, 82, 1961–1966. (11) Cheng, J.; Zhou, J.; Liu, J.; Cao, X.; Zhou, Z.; Huang, Z.; Zhao, X.; Cen, K. Powder Technol. 2004, 146, 169–175. (12) Folgueras, M. B.; Diaz, R. M.; Xiberta, J. Fuel 2004, 83, 1315– 1322. (13) Cheng, J.; Zhou, J.; Liu, J.; Zhou, Z.; Huang, Z.; Cao, X.; Zhao, X.; Cen, K. Prog. Energy Combust. Sci. 2003, 29, 381–405.
Reactions 3 and 5 are acid-base reactions that are thermodynamically favored at low and high temperatures.10 As shown in the reaction scheme above, the sulfur will be retained as CaS in the ash. CaS in its pure form is stable up to 2400 °C; nevertheless, its presence in the ash is not desired because it reacts with water at ambient temperature and releases the sulfur back to the atmosphere as H2S.14 Several publications have outlined a solution to this problem by transforming the CaS through reaction 7.14–18 CaS(s) + 2O2(g) T CaSO4(s)
(7)
The oxidization in reaction 7 is controlled by product layer diffusion when a certain layer of CaSO4 is formed. Reaction 7 might occur along with two other reactions 8 and 9. 3 CaS(s) + O2(g) T CaO(s) + SO2(g) 2
(8)
CaS(s) + 3CaSO4(s) T 4CaO(s) + 4SO2(g)
(9)
Because of reactions 8 and 9, the sulfur might be released back to the gas form as SO2 and, from that, lowering the retention efficiency of calcium. The optimum temperature for reaction 7 was found by ref 16 to be between 815 and 900 °C. The solid-solid reaction 9 was studied by mixing CaS(s) and CaSO4(s) and heating the mixture. They showed that, at 850 °C, no solid-solid reaction occurred, while at 1050 °C, a 100% conversion to CaO was achieved. Gas Cleaning with NTP. NTP treatment can operate at high temperatures and was proven successful in removing harmful compounds, such as tars19–24 and sulfuric25–29 and nitrous compounds.30,31 Most of the studies performed thus far have concentrated on the investigation of the removal efficiency of a single compound and some multiple compounds in a background gas that is usually either N2, H2, He, or Ar. Whether NTP is a viable solution for the gas cleaning of the gas produced (14) Qiu, K.; Lindqvist, O.; Mattisson, T. Ind. Eng. Chem. Res. 1998, 37, 923–928. (15) Hansen, P. F. B.; Dam-Johansen, K.; Østergaard, K. Chem. Eng. Sci. 1992, 48, 1325–1341. (16) Yrjas, P.; Hupa, M.; Iisa, K. Energy Fuels 1996, 10, 1189–1195. (17) Mattison, T.; Lyngfelt, A. Thermochim. Acta 1999, 325, 59–67. (18) Qiu, K.; Mattisson, T.; Steenari, B.-M.; Lindqvist, O. Thermochim. Acta 1996, 298, 87–93. (19) Jiang, C.; Mohamed, A. H.; Stark, R. H.; Yuan, J. H.; Schoenbach, K. H. IEEE Trans. Plasma Sci. 2005, 33, 1416–1425. (20) Nair, S. A.; Yan, K.; Pemen, A. J. M.; Winands, G. J. J.; Gompel, F. M.; Leuken, H. E. M.; Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. J. Electrost. 2004, 61, 117–127. (21) Nair, S. A.; Pemen, A. J. M.; Yan, K.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Plasma Chem. Plasma Process. 2003, 23, 665–680. (22) Mok, Y. S.; Nam, C. M.; Cho, M. H. IEEE Trans. Plasma Sci. 2002, 30, 408–416. (23) Pemen, A. J. M.; Nair, S. A.; Yan, K.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Plasmas Polym. 2003, 8, 209–224. (24) van Heesch, E. J. M.; Pemen, A. J. M.; Yan, K.; van Pasen, V. B.; Ptasinski, K. J.; Huijbrechts, A. H. J. IEEE Trans. Plasma Sci. 2000, 28, 1571–1575. (25) Ma, H.; Chen, P.; Ruan, R. Plasma Chem. Plasma Process. 2001, 21, 611–624. (26) Yankelevich, Y.; Pokryvailo, A. IEEE Trans. Plasma Sci. 2002, 30, 1975–1981. (27) Helfritch, D. J. IEEE Trans. Ind. Appl. 1993, 29, 882–886. (28) Kim, H. H.; Wu, C.; Kinoshita, Y.; Takashima, K.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 2001, 37, 480–487. (29) Zhao, G. B.; John, S.; Zhang, J.; Hamann, J. C.; Muknahallipatana, S. S.; Legowski, S.; Ackerman, J. F.; Argyle, M. D. Chem. Eng. Sci. 2007, 62, 2216–2227. (30) Okubo, M.; Inoue, M.; Kuroki, T.; Yamamoto, T. IEEE Trans. Ind. Appl. 2005, 41, 891–899. (31) Shang, K.; Wu, Y.; Li, J.; Li, G.; Li, D. Plasma Sources Sci. Technol. 2007, 16, 104–109.
Sulfur Abatement in Pyrolysis of Straw Pellets
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Table 1. Elemental Analysis of the Straw Pellets and the Solid Residues (Weight %, Dry Basis) strawc pellets pellets pellets pellets pellets P1-600 P2-600 P3-600 P4-600 P5-600 c
1 2 3 4 5
Ca
Ha
Na
Sa
46.00 45.65 45.38 45.49 45.17 45.05
6.20 6.09 6.08 6.06 6.04 6.05
0.63 0.76 0.73 0.74 0.71 0.72
0.11 0.072 0.068 0.064 0.062 0.059
0.11 0.11 0.11 0.11 0.11
0.117 0.126 0.132 0.114 0.124
Elemental Analysis 0.13 0.68 1.30 0.13 0.64 2.20 0.14 0.71 2.10 0.13 0.63 2.80 0.13 0.62 2.80
67.38 65.77 64.98 63.23 64.41
1.66 1.62 1.54 1.60 1.60
0.93 0.92 0.89 0.86 0.85
Sb
Cl
Ca
P
K
Ti
Mn
Fe
Zn
Br
Na
Mg
Al
Si
0.43 0.49 0.49 0.45 0.47
0.47 0.52 0.80 0.82 1.10 1.10
0.053 0.058 0.060 0.062 0.058 0.060
1.70 1.65 1.65 1.60 1.60
0.007
0.003
0.004 0.004 0.005
0.004 0.004 0.003 0.004 0.003
0.006 0.005 0.005
0.001 0.001
0.062 0.037 0.031 0.035 0.035
0.006
0.068 0.089 0.084 0.082 0.085
0.10 0.10 0.10 0.10 0.10
0.048 0.040 0.038 0.034 0.039
1.0 1.0 1.1 1.0 1.0
0.004 0.004 0.003 0.003 0.004
0.130 0.140 0.140 0.130 0.120
0.18 0.19 0.18 0.18 0.17
0.073 0.077 0.063 0.072 0.070
2.0 2.1 2.1 2.0 2.0
of Char 0.140 0.150 0.150 0.140 0.140
Residues after Pyrolysis at 600 °C 4.20 0.017 0.006 0.150 0.005 4.20 0.014 0.004 0.090 0.006 4.20 0.012 0.005 0.080 0.004 4.00 0.014 0.004 0.090 0.006 4.10 0.012 0.004 0.090 0.004
a Determination performed using an “EA 1108 CHNS-O” elemental analyzer by Carlo Erba Instruments. b Elemental analysis based on XRF. Elemental analysis of straw prior to pelletization (C/H/N, ASTM D5373; S, ICP-OES axial; Ca/P, ICP-OES radial).
from biomass gasification will not only depend upon its cleaning efficiency but also the energy needed for the plasma generation. When the balance gas of a NTP system is free from oxidants, the overall reaction of the decomposition of H2S becomes as follows: H2S(g) f H2(g) + S(s)
(10)
The theoretical energy requirement for the decomposition of hydrogen sulfide is 20.3 kJ/mol. Four reactions pathways that lead to the dissociation of H2S with a NTP were proposed in the literature:29 (1) direct ionization of H2S followed by dissociative neutralization, (2) ionization of the balance gas, leading to a charge-transfer reaction and subsequent dissociative neutralization, (3) dissociation through direct electron collision with H2S, and (4) electron collision with the balance gas, which produces active species that contribute to the dissociation of H2S. In this case, the active species can be either dissociated radicals or molecules in an excited state. In all of the proposed pathways, the outcome of the H2S dissociation is the radicals H and HS that react with each other and with other H2S molecules to form H2 and pure sulfur. Experiments performed with different balance gases resulted in the rejection of 1 and 2 as possible pathways for H2S dissociation.29 Furthermore, for experiments performed in this study, the fourth pathway is more likely to occur because of the lower concentration of H2S in the produced gas. In case of the use of N2 as a balance gas, dissociative reactions will occur through the excited state of N2. The dissociation of N2 to N radicals is less likely to occur.32 Because the processed gas in this study is based on the devolatilization of straw pellets, the dissociation of H2S might occur through radicals produced from other sources than the balance gas, for example, H radicals produced from the dissociation of H2. Experimental Setup and Equipment Sample Preparation. The samples were prepared at the Technical University of Denmark (DTU), Department of Mechanical Engineering, in a laboratory pellet mill used for testing and optimizing the pellet production of different solid fuels.33 The straw with its smooth and shiny surface has showed to be difficult to pelletize without the addition of a binding material. Suggestions of mixing a 10% calcium phosphorus solution (CAP) with the straw were considered. However, for this study, it was decided not to use CAP because further contamination of the elemental composition of the straw was not desired. Only 10% of water was added to (32) Zhao, G. B.; Garikipati, S. V. B. J.; Hu, X.; Argyle, M. D.; Radosz, M. Chem. Eng. Sci. 2005, 60, 1927–1937. (33) Holm, J. K.; Henriksen, U. B.; Hustad, J. E.; Sørensen, L. H. Energy Fuels 2006, 20, 2686–2694.
the straw. The water cools down the die temperature as it evaporates, while the pellets are being pressed. CaO and Ca(OH)2 were chosen as additives for the sulfur binding at the devolatization stage. The added quantity was chosen to produce a molar ratio of Ca/S of 2 and 4. This is the ratio of the calcium added through the additive relative to the sulfur found in the straw. The five different pellets are named P1-P5, with the following relation to additive type and amount: P1, no additives; P2, Ca/S ) 2 based on CaO; P3, Ca/S ) 4 based on CaO; P4, Ca/S ) 2 based on Ca(OH)2; and P5, Ca/S ) 4 based on Ca(OH)2. The ultimate analysis of the straw pellets is performed at the “Elsam Kraft A/S” laboratory in Denmark and is shown in Table 1. For the quantifications of C, H, and N, the American Society for Testing and Materials (ASTM) standard D 5373 was used. For S and Cl an inductively coupled plasma-optical emission spectroscopy (ICP-OES) technique was used. This table also includes the elemental analysis of the char produced after pyrolysis at 600 °C. Pyrolysis Reactor. The experiments were performed in a macrothermogravimetric analyzer (macro-TGA). The reactor is basically a vertical tube with an Al2O3 ceramic coating with an inner diameter of 100 mm and a length of 1000 mm. The core is insulated with five heating elements that can be temperature-controlled separately. The gas entering the reactor is preheated in a heating section situated at the entrance of the main reactor. The residence time of the pyrolysis products in the hot zone depends upon the reactor temperature and varies between 5 and 8 s. The setup of the reactor has been described in more details previously34 and is shown in the schematic drawing in Figure 1. Sampling Line. A partial flow for gas analysis was taken through an outlet near the top of the reactor. A quartz tube with an outside diameter of 6 mm and an inside diameter of 1 mm is used for the gas sampling. The sample gas passes first through an ice-cooled trap consisting of three glass bottles filled with glass wool. The sample gas is then led to another ice trap consisting of one steel condenser and another container filled with glass wool with a glass fiber paper filter on the top. The stream goes through a heated particle filter, after which it is led to two gas chromatographers and a Fourier transform infrared spectroscopy (FTIR) instrument. The quantification of H2S and COS was performed with a Varian CP-4900 micro-gas chromatograph (micro-GC) with a Pora-plot column attached to a differential mobility detector (DMD). The quantification of other compounds was performed with a Bomem 9100 FTIR and a Varian CP-4900 micro-GC with a thermal conductivity detector (TCD). The sampling line is shown together with the macro-TGA in Figure 1. A coaxial dielectric barrier discharge (DBD) reactor was used to treat the devolatilized products. A detailed description of the NTP reactor along with the effect of the NTP on the major devolatilized products can be found (34) Becidan, M.; Skreiberg, Ø.; Hustad, J. E. J. Anal. Appl. Pyrolysis 2007, 78, 207–213.
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Khalil et al. performed with the presence of the NTP reactor. These were performed with the same pyrolysis temperatures but only with the straw pellets free from additives (P1). The setup of the tests is as follows: (i) with NTP reactor placed upstream to liquid condensation, macro-TGA flushed with nitrogen, (ii) with NTP reactor placed downstream to liquid condensation, macro-TGA flushed with nitrogen, and (iii) with NTP reactor located upstream to liquid condensation, macro-TGA flushed with nitrogen and carbon dioxide (CO2/N2 mol ratio of 17:8). The aim of these experiments was to investigate the influence of plasma treatment on the abatement of the sulfur compounds in the devolatilized products. When experiments were performed both up- and downstream of the condensation train, it was possible to investigate the removal efficiency of the sulfur compounds both with and without the presence of the condensable fraction in the processed gas. The aim of the last experimental set is to investigate the role of the background gas on the product distribution. Having a high concentration of CO2 in the background gas will produce CO and active oxygen by the dissociation of CO2 according to the following formula:
CO2 + e- f CO + O-
(11)
The increased availability of active oxygen might change the chemical reaction path of sulfur.
Experimental Results
Figure 1. Schematic diagram of the reactor and the gas sampling line. Table 2. Properties of the NTP Reactor reactor type primary voltage secondary voltage reactor material outer electrode length plasma gap width residence time of gas in the plasma region frequency plasma power
coaxial dielectric barrier discharge 55 V 33 kV quartz cylinder with a brass electrode in the middle 10 cm 2.8 mm 0.12 s 1500 Hz 100-150 W
elsewhere.35 A brief description of the NTP reactor properties is reproduced in Table 2.
Experimental Procedure Only isothermal experiments were performed in this work. The reactor was preheated to the desired temperature and, at the same time, purged with nitrogen (25 nL/min) for about 90 min. The pellets were kept at 105 °C for at least 24 h prior to the experiment. A total of 80 g of sample material was put inside the basket and attached to the scale. Once the oxygen concentration inside the reactor was down to a satisfactory level, the sample basket was rapidly lowered into the reactor. The experiment was stopped when the gas analyzer was no longer able to detect any major gas compounds. Experiments were performed with all of the pellets produced (P1-P5) and for pyrolysis temperatures of 400-800 °C, with steps of 100 °C. In addition, three sets of experiments were
Effect of the Calcium-Based Additives. The effect of calcium-based additives seems to have only a marginal influence on the retention of sulfur. The additive influence on the sulfur compounds found in the devolatilized products was marginal and could not be differentiated from the experimental uncertainties. These results were therefore omitted, and conclusions on the effect of additives will be drawn from Table 1. This table shows the elemental analysis of the straw pellets and the char fraction after pyrolysis at 600 °C. These results were reported from experiments performed in three different laboratories using different methods for the quantification of the elemental composition. The sulfur content is reported in two different columns and is based on two different methods. The column marked with footnote b uses X-ray fluorescence spectroscopy (XRF). This method is based on hitting the sample with X-rays, R particles, protons, or high-energy electrobeams. By doing so, the atom absorbs the energy, resulting in an ejection of an electron from the inner shells to an outer shell. The unstable atom goes back to its original state and releases energy that can be picked up and measured by a detector. Because each element has a unique set of energy levels, a quantitative and qualitative measurement can be performed without the need of destroying the sample. The column marked with footnote a is produced using a thermal method, which consists of heating the sample in a pure oxygen environment at a temperature of 1020 °C. The oxidized products pass through a catalyst reducing the SOx formed to SO2. The combusted products are usually measured by a GC. The XRF method reports a constant concentration of sulfur for all pellets types 1-5. The thermal method yielded a lower concentration level, leading to the fact that one can believe that a portion of the sulfur has been retained in the ash. Furthermore, increasing the Ca/S in the pellets has resulted in more sulfur retention, although the effect is marginal. This means that under oxidative conditions and at temperatures as high as 1020 °C, the calcium-based additives are still able to retain some of the sulfur in the ash. The results for the elemental composition of the char rest after pyrolysis at 600 (35) Khalil, R.; Seljeskog, M.; Hustad, J. E. Energy Fuels 2008, 22, 686–692.
Sulfur Abatement in Pyrolysis of Straw Pellets
Figure 2. Removal efficiency of 42 ppm of H2S in pure N2 as a function of NTP power.
Figure 3. Removal efficiency in pure N2 as a function of the H2S concentration.
°C seems to yield a constant sulfur content for the XRF method, with some hints of increasing values for P3-600. For the thermal method, the values are increasing with increasing Ca/S ratios, which means the additives play a positive role on retaining the sulfur during the pyrolysis. The sulfur in the char matrix after pyrolysis at 600 °C has been retained at increasing levels with an increasing calcium concentration. Plasma Removal Efficiency. Prior to experiments with the devolatilized products, the NTP reactor was used to test the dissociation of H2S in pure N2. For these experiments, a calibration gas containing 206 ppm H2S in N2 was diluted with N2 to vary the H2S concentration. The gas flow was controlled with flow controllers and mixed together in a special mixing chamber designed at our laboratory. The new concentration was directed to the NTP reactor, and the concentration of H2S was measured right after the plasma region. The NTP geometry, residence time of the gas in the plasma region, and the gas flow were chosen to be equal to the experiments performed on the devolatilized products. Both the concentration of H2S and plasma power was varied. The results are shown in terms of removal efficiency in Figure 3. The power of the plasma was varied by increasing the primary voltage from 45 to 70 V, with 5 V increments. In Figure 2, the corresponding secondary voltage is shown on the x axis. The power of the plasma is also shown for the individual points, except for the last two points. The initial concentration of H2S for this experiment was kept constant at 42 ppm. For the experiment shown in Figure 3, the plasma power was kept constant and at a primary voltage of 60 V, while the H2S concentration was increased. From both figures, we can see that the removal efficiency of H2S is quite
Energy & Fuels, Vol. 22, No. 4, 2008 2793
Figure 4. Total formation of H2S relative to the dry straw pellets.
high and increases with both increasing power and H2S concentration. During and after these experiments, solid sulfur was observed to build up as a dust layer on the inner surfaces of the outlet tubing after the NTP reactor. During the pyrolysis experiments, three significant sulfuric compounds are expected to be found in the devolatilized gas products: H2S, COS, and SO2. The quantification of SO2 was found to be the most challenging because compounds, such as H2O, C2H2, and CH4, interfere heavily at the wavenumber where SO2 should appear when using the FTIR instrument. The spectra generated during the experiments were manually treated, and single compound calibration spectra with the appropriate concentration were subtracted from the original spectra to obtain a less disturbed SO2 profile. Attempts made upon detecting SO2 for all temperatures have shown that SO2 was below the detection limit of 10 ppm for the FTIR. As explained earlier, a micro-GC with a DMD detector was used for the quantification of H2S and COS. This type of detector is quite sensitive and has a lower detection limit of 0.1 ppm for both H2S and COS. The detector is tuned by the producer to selectively detect sulfuric compounds and does not have any interference problem. However, stability problems resulted in a daily calibration of the instrument. The calibration was performed before and after the experiments by purging calibration gas containing first H2S and later COS through the sampling system shown in Figure 1. The calibration procedure was performed each time in a similar manner in terms of the order of the concentrations and the calibration gas type. The releases of sulfur through the pyrolysis of straw pellets for all of the experiments are shown in Figure 4 for H2S and Figure 5 for COS. The results are shown as a function of the pyrolysis temperature in milligrams of H2S per kilogram of dry straw pellets. The sulfur release was dominated by the formation of H2S, while COS was present at a much lower concentration. For all experiments, the trend of H2S release increases with an increasing pyrolysis temperature. For the experiments where no plasma was used, the integrated amount of H2S is ca. 149 mg/kg straw pellets for the pyrolysis temperature of 400 °C and ca. 314 mg/kg straw pellets for 800 °C. This corresponds to a sulfur weight percentage of respectively 12.7 and 26.8% relative to the sulfur in fuel. Figure 4 shows clearly the positive effect of plasma on the removal of H2S. For experiments where the NTP is placed upstream to liquid condensation, the removal efficiency exceeds 50% at a pyrolysis temperature of 400 °C, while for other temperatures, the removal efficiency is close to 45%. Placing the NTP reactor downstream of liquid condensation, clearly improves
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Figure 5. Total formation of COS relative to the dry straw pellets. Table 3. RSD of Sulfur Compounds, Performed at 500 °C and with the NTP Placed Upstream of Liquid Condensation repetition 1 repetition 2 repetition 3 RSD (%)
H2S
COS
73.0 74.9 52.4 18.7
5.0 3.2 3.0 28.8
the H2S removal efficiency compared to the upstream position. This can be explained by the fact that the producer gas is less complex because it does not contain the condensable fraction. The advantage of placing the NTP reactor downstream to the liquid removal unit diminishes at higher pyrolysis temperatures. This is because the producer gas at higher pyrolysis temperatures contains less condensable liquids. As mentioned earlier, the last set of data points in Figure 4 are performed with a carrier gas containing a higher CO2 concentration. These experiments have the same location of the NTP reactor as the second set and can therefore be compared. Relative to the use of pure N2 as a carrier gas, it is evident that the increase of the CO2 fraction in the producer gas is playing a positive role on the H2S removal efficiency. It has been shown in an earlier publication35 that CO2 during the influence of the plasma will produce CO and oxygen radicals. These oxygen radicals are contributing to a higher degree of H2S abatement. The outcome of these reactions is an increased formation of COS, while SO2 remains under the detection limit. It is important to point out that the measured COS concentrations are quite low, which makes the uncertainties relatively high. The reproducibility for both H2S and COS have been investigated by performing three repetitions of the experiment at 500 °C, with the NTP reactor placed upstream relative to liquid condensation. The relative standard deviation (RSD) is presented in Table 3 along with the actual experimental data. The high deviation values are due to the sum of several sources of uncertainties. The most important ones are the low concentration of the measured sulfur species and the fact that we are measuring a dynamic concentration profile with a micro-GC that samples at a 2 min interval. The later point affects the results during the integration of the concentration profile because a linear interpolation between the sampling points is assumed. Figure 6 gives an idea on the shape of the concentration profile of H2S at a 400 °C pyrolysis temperature. Nevertheless, the trends in Figures 4 and 5 are quite clear, and all of the commented differences are well within the measuring uncertainties. The trends for COS formation are similar to H2S with some important differences. In comparison to H2S, the
Figure 6. H2S concentration profile for experiments at a pyrolysis temperature of 400 °C.
COS formation decreases with an increasing pyrolysis temperature but increases when CO2 is introduced with the carrier gas. The measured COS concentrations are quite low for all of the experiments performed at 800 °C, which leads to difficulties interpreting the results at that particular temperature. Conclusions Danish straw has been mixed with the calcium-based additives CaO and Ca(OH)2 at different Ca/S ratios. The mixed straw was pelletized and pyrolized at different temperatures in a macro-TGA reactor to study the sulfur retention in the ash. A detailed analysis of the sulfur species in the gas products has been presented along with the effect of a NTP on the abatement of sulfur release. The NTP reactor was used for treating a flow of 5 nL/min taken from the macro-TGA for the quantification of the devolatilized products. The NTP treatment was performed in three different modes while keeping all of the geometric and electric properties of the DBD constant. The main findings are as follows: (1) The sulfur release with the pyrolized products increased with increasing temperatures. The effect of the additives could not be determined accurately by solely studying the composition of the evolved gases. When an elemental analysis of the straw pellets and the solid residues was performed after pyrolysis at 600 °C, some tendencies toward increased sulfur retention could be observed, although this effect appeared to be marginal. Data from literature reporting the influence of the same type of additives on the pyrolysis of coal were more noticeable. This is most likely due to the higher content of sulfur in coal compared to straw. Another major difference between coal and straw is the higher ash content in the later fuel, which also has a high calcium content. (2) Increasing the pyrolysis temperature has resulted in an increased release of H2S in the devolatilized products. COS formation decreased with an increasing pyrolysis temperature. (3) The NTP reactor has been proven to have a high H2S removal efficiency. In pure N2 and at a concentration of 42 ppm, the maximum removal efficiency was close to 95%. Increasing the H2S concentration to 165 ppm has resulted in improved removal efficiency. (4) The removal efficiency of H2S in the pyrolysis experiments was highest when the NTP reactor was placed downstream of liquid removal and at a pyrolysis temperature of 400 °C. At this configuration, 86% of H2S was removed from the devolatilized products. Best plasma reactor placement was proven to
Sulfur Abatement in Pyrolysis of Straw Pellets
be downstream of liquid removal for both H2S and COS. (5) Increasing the CO2 amount in the carrier gas has improved the removal efficiency of H2S at the cost of increased COS formation. Future Work. Work has already started on mixing new types of additives with the straw pellets. This work is being performed by a new Ph.D. student, where the effect of additives on the gasification kinetics, the improvement of the ash melting point, and the sulfur release will be addressed. Work will also continue on the improvement of the NTP reactor. Future experiments
Energy & Fuels, Vol. 22, No. 4, 2008 2795
will include better plasma efficiency and the possibility to run the plasma reactor at high temperatures. Acknowledgment. This work was supported by The Research Council of Norway through the program “Energy for the Future”. The authors thank Jens Holm at the Technical University of Denmark for his help during our work on the pellet production. The authors also thank the Master’s students Harris Utne and Monica Moen for their valuable help during the experimental work. EF8001235
